Low temperature propagation and detection of high frequency acoustic phonons
in GaAs and InAs is studied. .Phonon imaging is used to study scattering of acoustic
phonons in GaAs and angular dispersion of acoustic phonons in InAs. Photoluminescence
spectroscopy is used to study the effects of nonequilibrium phonons on excitons in
epitaxial semiconductors.
The propagation of high frequency phonons through crystals at low temperatures
is characterized by both ballistic and diffuse processes. Ballistic propagation of heat
pulses is highly anisotropic due to phonon focusing. Experimental data is compared to
Monte Carlo calculations and it is shown that the heat flux from phonons scattered a few
times in the bulk retains a significan~ degree of anisotropy. The scattering rate of 0. 7
·.,
THz phonons in GaAs is measured and a difference between the scattering of longitudinal
and transverse modes is reported.
The phonon focusing pattern changes with increasing phonon frequency due to
angular dispersion. The results of frequency-selective phonon-focusing experiments on
InAs are compared to Monte Carlo calculations incorporating Rigid Ion and Bond-Charge
lattice dynamics models. The Bond-Charge model more accurately predicts the evolution
of the phonon-focusing pattern in the frequency range investigated. Predictions of the
focusing pattern for very high frequency phonons ( v > 1012 Hz) are presented.
The intensity of lines associated with free and impurity-bound excitons in the
photolu~nescence spectrum of epitaxial GaAs changes with temperature even for T <
2°K. Similar spectral changes can be caused by nonequilibrium phonons created by a
laser focused on the substrate crystal face. Experiments demonstrating the detection of
nonequilibrium phonons propagating ballistically through the substrate are described.
The possibilities of using this system as a detector for phonon-imaging experiments is
discussed.